The efficiency of solar heat collectors is affected by the amount of convection, conduction and radiation losses associated with their construction. It has been found that heat losses due to convection and conduction may be materially reduced by evacuating the air space within the solar collector about the absorber or collector plate. However, in the case of standard flat plate solar heat collectors having standard sized flat plate absorbers, it has not been possible to evacuate such collectors and provide a single solar window over the same which is supported solely about edge portions thereof, due to the atmospheric load which is exerted thereon upon evacuation of the collector. That is, with a standard atmospheric pressure of about 15 lbs./sq. in., an evacuated relatively shallow solar collector structure will have an atmospheric pressure of approximately one ton per square foot on the collector window.
Realizing that it was impossible to subject the standard flat window of a flat plate heat collector to such forces without catastrophic failure, the prior art devices such as shown in U.S. Pat. No. 3,929,122 and No. 3,974,823 utilized two solar windows spaced apart from one another to form a dead air space between the ambient atmosphere and the interior of the solar collector containing the absorber plate. Although the dead air space provided a degree of insulation, convection and conduction heat losses of significant magnitude were still experienced.
In order to provide the desired vacuum within the solar collector so as to minimize convection and conduction heat losses from the absorber plate through the collector window to the ambient atmosphere, tubular solar collectors were utilized as shown in U.S. Pat. No. 3,227,153. The use of the tubular construction, which is strong in compression, permitted the evacuation of the solar collector and thereby materially reduced heat losses due to convection and conduction. However, the diameter of the solar collector was of course limited to practical aspects which accordingly limited the area of the flat plate collector or absorber member retained therein. Thus, in order to obtain the same surface area as was obtainable with a standard flat plate collector of a relatively large shallow structure, it was necessary to provide a multiplicity of such tubular collectors.
Other attempts have been made in supporting expansive flat solar windows in evacuated flat plate collectors, such as utilizing support posts as shown in U.S. Pat. No. 3,995,615 and longitudinally extending partition walls as shown in U.S. Pat. No. 4,038,965. Although the support posts of the former patent permit utilization of a standard flat plate collector, each of the support posts in fact reduces the effective area of the collector and may impart unsafe stress to the flat window at each contact point. In the case of the latter patent, the plurality of longitudinal partitions necessitate the utilization of a plurality of small collector plates similar to that used in the evacuated tubular collector, and again such partitions may interfere with optical efficiency and function as conductors or heat sinks to the solar window with the resultant loss of efficiency.
U.S. Pat. No. 3,986,491 discloses the use of a sheet of transparent or translucent corrugated plastic positioned above and across a metallic heat collecting surface having hills and valleys, with the corrugation as far as possible focusing the rays of the sun on one side of the flaring hills in the morning and focusing the solar rays on the opposite side of the hills during the afternoon. However, the solar collector is not evacuated, as an air space is provided within the collector between the transparent or translucent plastic solar window and the metallic collector plate.
U.S. Pat. Nos. 4,186,723 and 4,184,480, the teachings of which are incorporated herein by reference, disclose in various embodiments evacuated insulating contoured windows which are supported solely about a peripheral seal edge for the structure. A plurality of laterally extending support beams project arcuately outwardly from the support edge and are joined along adjacent portions to form a plurality of sinusoidal corrugations. In the former a solar panel within the window is in contact with only a limited portion of the seal edge and thus conduction losses are minimized. While the above structure may be constructed having any length desired, the lateral dimension is limited to practical size by the contour of the beams.
Accordingly, the present invention has overcome the problems of prior art devices by providing a perforated or segmented evacuated flat plate collector within a substantially continuous solar window which is supported about peripheral portions and intermediate portions within the perforations which withstand the atmospheric forces exerted thereon.